Why can't an unsaved person understand spiritual truths?
The unsaved person cannot understand spiritual truths because they are spiritually discerned. Scripture says the natural, unconverted, unregenerated man receives not the things of the Spirit of God; they are foolishness to him, and he cannot know them until God opens his eyes and heart.
Les teaches that the unsaved cannot grasp spiritual truth because such things are spiritually discerned. In 1 Corinthians 2:13-14, Paul makes it plain that he speaks not in words of man's wisdom but in words the Holy Spirit teaches, comparing spiritual things with spiritual. The natural man—the unconverted, unregenerated, unsaved, lost man—receives not the things of the Spirit of God. They are foolishness to him, and he cannot understand or know them. Les adds that believers must be patient with unbelievers, recognizing they cannot comprehend these things until God opens their eyes and hearts, leaving that to His own good timing.
But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned.